1. Field of the Invention
The present invention relates generally to a method for preventing galvanic corrosion of pipe and, more specifically, to a method for coupling carbon steel pipe to Corrosion Resistant Alloy (CRA) pipe, where the threads within the couple are protected in an oxygen free environment, the method also including the application of an abrasion resistant nonconductive coating to the surface of the CRA pipe, particularly where the pipe is used downhole and the couple is submerged in an electrolytic fluid.
2. Description of the Prior Art
For many years, the chemical, petrochemical, oil and gas, sulfur and related industries were faced with the problem of galvanic corrosion in underground wells and associated surface piping, which problem was intensified by electrolytic fluids encountered in normal well operations. Exposure of steel pipe, tubing and casing to fluids in this environment decreased the useful life of the materials and increased maintenance costs associated with the wells. A casing leak in a waste injection well could require installation of a repair liner or plugging of the well. In cases where an oil or gas well exhibited only marginal production, the increased maintenance cost could determine whether or not the well was kept in production.
Whenever two dissimilar metals are connected, a galvanic couple is created. This couple will result in corrosion of the anode, the less noble of the two metals making up the couple. The severity of the reaction between the two metals is determined by several factors including the electromotive nature of each metal and the galvanic potential difference between the two. Another primary factor is the environment surrounding the galvanic couple including the conductivity, temperature and oxygen concentrations within the well bore or other environment.
In order to reduce installation costs, some waste injection wells combine carbon steel with corrosion resistant alloy (CRA) casing, tubing and piping materials. The carbon steel material is utilized in the upper portion of the pipe string in the well above the injection interval and CRA is used for the lower portion from just above the packer to the base of the pipe string. The connection of the carbon steel to the CRA creates a galvanic couple. The resulting galvanic reaction accelerates corrosion of the carbon steel, leading to premature failure immediately adjacent the connection of the two dissimilar metals unless corrective action is taken.
Galvanic corrosion can be halted when the flow of electrical current from the cathode (CRA) to the anode (carbon steel) is eliminated. A common method of eliminating the current flow has been the use of a non-metallic section of pipe, such as fiberglass, between the carbon steel and the CRA. U.S. Pat. Nos. 2,950,928; 3,185,501 and 3,346,274 all show electrically insulating joints for connecting pipes, casing and tubing in a well pipe string. All of these connecting joints utilize at least one resilient insulating member to prevent physical contact between the pipes. This construction renders it impractical to use such a system in the high temperature or corrosive environment of many injection wells or oil or gas wells due to the rapid deterioration of the resilient element. Also, the inferior pullout strength of these joints would prohibit their use in most well strings or similar piping arrangements. Other insulating pipe couplings are shown, for example, in U.S. Pat. No. 3,871,687. However, none of the above couplings are designed for use in high temperature or corrosive well environments. Thus, the use of a non-metallic section of pipe to halt galvanic corrosion has certain inherent drawbacks including the inability to maintain a seal in the connection between the non-metallic and metallic casing and the lack of durability while drilling out or running tools through the non-metallic pipe.
Another method which has been attempted for controlling galvanic corrosion is to apply a protective coating to the less noble or carbon steel pipe. This method is potentially worse than applying no coating to either section of pipe. There is a high probability that, either due to defect during the coating application or damage during installation, maintenance or operation of the well, a small area will become uncoated. The entire galvanic current projected from the CRA cathode to the carbon steel anode is then concentrated on the small, uncoated area accelerating corrosion at that location.
While the entire well pipe string can be formed of a CRA material such as stainless steel or other corrosion resistant material, this solution is generally uneconomical due to the much higher cost of such materials. Obviously, the more viable solution is to use the CRA material, such as stainless steel, only through the most corrosive area of the well operation and to join it to a string of carbon steel pipe which is used to make up the remainder of the pipe string. However, as discussed above, the corrosive fluids present in many well environments offer the ideal conditions for galvanic corrosion between the dissimilar metals. The result is a faster corrosive attack of the carbon steel piping and a slower attack of the stainless steel as compared with the same materials when they are not in contact.
A need exists, therefore, for a method to couple pipe which effectively protects two adjacent sections of pipe of dissimilar metals included in a string of well pipe.
A need also exists for such a method to couple pipe which is simple in design and economical to manufacture.
Another object of the invention is to provide such a method to couple pipe as well as an internal coating system for use in electrolytic environments which is capable of withstanding the abrasive forces associated with the operation of drilling and downhole tools within the pipe string without significant loss of coating area while containing galvanic electrical current.
Another object of the invention is to provide such a method to couple pipe which method provides the durability required to allow drilling and associated downhole tools to operate within the pipe string without compromising the integrity (tensile and compressive strength, hydrostatic resistance, etc.) necessary for satisfactory performance throughout the design lifetime of the well.
Another object of the invention is to provide a method to couple pipe with galvanic corrosion protection which offers increased resistance to "pull out" so as to enable the couple to support the pipe, tools and appliances that may be suspended below it in a subterranean well bore.